This invention relates to ultrasonic apparatus primarily used for wire bonding but also used for various other processes such as semiconductor chip and input/output (I/O) pin removal. More particularly, this invention relates to an apparatus for monitoring the adhesion of the wire during bonding and the dehesion of a semiconductor chip and I/O pin during removal.
The process of ultrasonic bonding is well known. Briefly, in ultrasonic bonding, the bonding tool is caused to vibrate at ultrasonic frequency while the wire end is applied under pressure to the portion of the body to which electrical connection is to be made.
A recognized problem of ultrasonic bonding is that the bond quality can vary appreciably from bond to bond on a given substrate even where bonding machine settings are nominally identical. One very important factor involved in the bond quality is the bond strength. In some cases, low strength bonds can be identified visually such as where there are defects apparent. However, in most cases no estimation of the strength of the bond can be made without destructively pulling or shear testing the bond. This is not very practical since all bonds cannot be tested and the few marginal bonds present a serious reliability problem. At the present time, there are no commercially available ultrasonic wire bonding apparatus having provisions for the in-situ monitoring the bond quality and/or strength.
Landes U.S. Pat. No. 4,341,574 discloses an ultrasonic wire bond monitoring apparatus which determines the electrical impedance of the ultrasonic generator and then, based on the second derivative of the impedance, turns the bonding tool on or off. The monitoring apparatus can also determine when there is no wire in the bonding tool and when the tool is not making contact with the wire.
Chan et al. U.S. Pat. No. 4,606,490 discloses an ultrasonic wire bond monitoring apparatus which measures the current from the ultrasonic generator over time to generate a current envelope. The current envelope is compared with previously determined current envelopes to determine if a good bond is being made.
The current state of the art approaches to the problem have thus far failed to reliably distinguish bad bonds from good bonds. The present inventors have, therefore, proposed an apparatus to reliably detect when a good ultrasonic bond has been made.
The use of an ultrasonic apparatus to remove or debond parts is less well known. Behun et al., "Ultrasonic Method to Remove Chips Mounted on Ceramic or of Other Materials Substrates", Research Disclosure, No. 328 (August, 1991), the disclosure of which is incorporated by reference herein, discloses the ultrasonic removal of chips from a ceramic substrate. Basically, a chip remove stud is adhered to the backside of a chip. The chip has been previously joined to the ceramic substrate via solder controlled collapse chip connections. The chip remove stud is attached to the horn of the ultrasonic tool. Upon the application of ultrasonic energy and an upward force, the solder connections break and the chip becomes debonded or dehered.
A problem with this method of chip removal, as discovered by the present inventors, is that as more of the solder connections become broken, the chip begins to vibrate to a greater extent. This follows from the fact that as the solder connections become broken, a constant amount of ultrasonic energy is being applied to a decreasing number of solder connections. Consequently, when the last of the solder connections are broken, the chip abruptly debonds or deheres from the substrate with frequent damage to the chip and the substrate.
The present inventors, therefore, have proposed an apparatus to monitor the adhesion of the chip to the substrate so that upon detecting a lessening of such adhesion, the power to the ultrasonic tool can be lessened to alleviate the abrupt debonding or dehering of the chip from the substrate.
It should be understood that the terms dehere and dehesion mean the opposite of adhere and adhesion and indicate the debonding or separating of components from a substrate.
Similar considerations apply to the abrupt removal of I/O electrical connector pins from a substrate.
It is, accordingly, a purpose of the present invention to have a monitoring apparatus to reliably detect when a good ultrasonic bond, for example between a wire and a substrate, has been made.
It is another purpose of the present invention to have a monitoring apparatus that regulates and varies the ultrasonic power according to the current adhesion conditions so as to perform chip removal, pin pull and the like without undue harm to the substrate.
These and other purposes of the invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.